The only law we have against tattoos is the verse that for ids drawing on your skin or making cuttings for the dead. This of course was under Judaic law, and even then was not even required for righteous Gentiles. It was for Jews alone. So it is safe to say it is irrelevant today, the only people who would even debate it are Messianic Jews.
So now knowing that it is lawful to us we must ask this: Is it a good idea?
I never liked them, but that is my taste. I actually had a plan for if I were to get one, but my wife does not like them and I don't like them on her so we never did.
It would be a Cross on my torso, left of center, made up of pictures of important moments in my life (conversion, marraige, etc.) on one bar, and important moments in our faith in the other. (Moses and ten commandments, Crucifixion, and ending with the second coming, and judgement)
The Dante's inferno game cross scarification was my inspiration for it.